COMMENTATIO ASTRONOMICA. f209 



« rfisolved. In giving us ihe opiuion which Ihe doclor had of Ihe result 6f his own ob- 

 "« servations with regard to Ihe annual parallax , de la Lande only menlions ; « M. 

 « « Bradley pense que si eile (la parallaxe ) eüt ete seulement de 1", il l'auroit ap- 

 « « percu dans le grand nombre d'observations qu'il avoit faites , surtout de y du Dra- 

 K « gon." But if we also take in Ihose lines upon which Dr. Bradley seems to lay 

 « the grealest stress, viz. « I believe I may venture to say , Ihat in either of the two 

 ■« K Stars last mentioned it does not amount to two seconds;" and if we allow for the 

 « magnitude of the slars upon which the observations were made , I Ihink I have fairly 

 « slated the füll amounf of all the actual proofs we have of the smallness of the annual 

 « parallax. Now since it has escaped Ihe finest observations of Bradley, it is not like- 

 « ly that it should come up to the füll quantily to wliich it might araount wilhout bein" 

 « perceived ; and therefore the Doclor might think it highly probable , « that it is not so 

 « « great as one single second;" and his opinion as well as de la Lande's, who be- 

 « lieves it to be absolutely insensible , are perfectly consislent with all the observations 

 « that have hilherto been made : though the actual proofs , which are the subject of 

 « our present inqniry, do not extend so far. Against the parallax of Sirius de la 

 «Lande, art. 2781, menlions, « fortyfive meridian allitudes taken by Dr. Bevis, 

 « « [Note. These observations were not made by Br. Bevis, but extracled from the 

 « <( registers of the Royal observatoyy at nty desire , and calculated by myself , and 

 « « sent in a letter by Dr. Bevis to Paris," Nevil Maskelyne, ] with the eight- 

 « « feet mural quadrant of the Royal Observatory at Greenwich , none of which differed 

 <■ « 3" or 4" from the mean altilude. " Now if ihey differed 3" or 4" from the meani 

 « we may suppose Ihey differed 6" or 8' from each olher ; and that observations sub- 

 « ject to so many ca^ses of error as I shall presenlly enumerate , and which differed so 

 .« much from each olher, cannot give tlie least evidence eilher for or against a paral- 

 « lax, will need no proof. Refraction alone, which is liable to such changes at the 

 «meridian altilude of Sirius, notwithslanding the most carefui observations of Ihe ba- 

 « romeler and Ihermometer should be made to ascertain its quanlily , would , with me , 

 « remain an unanswerable argument against the validity of such observations ii) a sub- 

 « ject of this crilical nicely. ] " 



« In general the method of zenilh dislances labours under the follow'Hg considerable 

 « dilEculties. In the first place, all these dislances, though Ihey should not eiceed a ' 

 « few degrees, are liable to refractions ; and I hope to be pardoned when I say that 

 « Ihe real quanlities of ihesc refractions, and their differences, are very far from beiag 

 « perfectly known. Secondly Ihe change of position of the earlh's axis arising from nu- 

 « talion , precession of the equinoxes , and olher causes , is so far from being coropletely 

 « settied , that it would not be very easy lo say what it exactly is at any given time. 



Dd « la 



